Always use the dark window borders

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -788,39 +788,15 @@ void NonClientIslandWindow::_UpdateFrameMargins() const noexcept
         return ret;
     }
 
-    // Set the frame's theme before it is rendered (WM_NCPAINT) so that it is
-    // rendered with the correct theme.
-    _UpdateFrameTheme();
+    // This is a hack to make the window borders dark instead of light.
+    // It must be done before WM_NCPAINT so that the borders are rendered with
+    // the correct theme.
+    // For more information, see GH#6620.
+    LOG_IF_FAILED(TerminalTrySetDarkTheme(_window.get(), true));
 
     return TRUE;
 }
 
-// Method Description:
-// - Updates the window frame's theme depending on the application theme (light
-//   or dark). This doesn't invalidate the old frame so it will not be
-//   rerendered until the user resizes or focuses/unfocuses the window.
-// Return Value:
-// - <none>
-void NonClientIslandWindow::_UpdateFrameTheme() const
-{
-    bool isDarkMode;
-
-    switch (_theme)
-    {
-    case ElementTheme::Light:
-        isDarkMode = false;
-        break;
-    case ElementTheme::Dark:
-        isDarkMode = true;
-        break;
-    default:
-        isDarkMode = Application::Current().RequestedTheme() == ApplicationTheme::Dark;
-        break;
-    }
-
-    LOG_IF_FAILED(TerminalTrySetDarkTheme(_window.get(), isDarkMode));
-}
-
 // Method Description:
 // - Called when the app wants to change its theme. We'll update the frame
 //   theme to match the new theme.
@@ -833,7 +809,6 @@ void NonClientIslandWindow::OnApplicationThemeChanged(const ElementTheme& reques
     IslandWindow::OnApplicationThemeChanged(requestedTheme);
 
     _theme = requestedTheme;
-    _UpdateFrameTheme();
 }
 
 // Method Description:

src/cascadia/WindowsTerminal/NonClientIslandWindow.h

@@ -85,7 +85,6 @@ class NonClientIslandWindow : public IslandWindow
     void _UpdateFrameMargins() const noexcept;
     void _UpdateMaximizedState();
     void _UpdateIslandPosition(const UINT windowWidth, const UINT windowHeight);
-    void _UpdateFrameTheme() const;
 
     void _OpenSystemMenu(const int mouseX, const int mouseY) const noexcept;
 };